Relative pressure measuring transducers are applied for registering relative pressures in many fields of industrial measurements technology.
In such case, the reference pressure is, as a rule, an ambient pressure in the environment of the measuring transducer. In the case of the most applications, this is the atmospheric pressure at the location of use. If, however, the measuring transducer is applied e.g. in a space under positive- or negative pressure, then the reference pressure is, for example, the internal pressure of the space.
Relative pressure measuring transducers contain a relative pressure sensor, which is arranged in a housing and supplied with both the pressure to be measured as well as also the reference pressure.
Relative pressure sensors usually comprise a pressure measuring chamber sealed by a pressure sensitive element, especially a measuring membrane. In measurement operation, the outside of the pressure sensitive element is supplied with the pressure to be measured, while its inner side is supplied with the reference pressure via a reference pressure supply communicating with the pressure measuring chamber. The resulting deflection of the pressure sensitive element dependent on the relative pressure is registered by means of an electromechanical transducer and converted into an electrical signal dependent on the relative pressure. The electrical signal is then available for additional processing and/or evaluation.
A disadvantage of such measuring transducers is that a connection to the environment is present via the reference pressure supply, so that moisture from the environment can get into the measuring transducer, especially into the relative pressure sensor.
In such case, the moisture gets into the measuring transducer via the air volume exchanged with the environment in the case of pressure transfer via the reference pressure supply. If the temperature in the environment of the measuring transducer is higher than the temperature in the interior of the measuring transducer, then the dew point can be subceeded in the interior of the measuring transducer, and condensate forms.
If moisture gets into the interior of the reference pressure sensor or in contact with the electromechanical transducer, then this can lead to significant degrading of the accuracy of measurement and in the extreme case even to the complete failure of the measuring transducer.
This problem can be defended against, on the one hand, by trying to avoid penetration of moisture into the measuring transducer. To this end, European patent, EP 1 070 948 A1 describes using a hydrophobic filter in the opening of the measuring transducer, with which the reference pressure supply communicates. The filter does indeed have some effect in protecting the inner space against moisture. However, penetration of moisture cannot be completely suppressed, due to the required air permeability. Especially when the measuring transducer is exposed to a hot environment with high humidity, humid air gets into the housing, so that upon a later cooling a residual moisture remains in the housing.
For solution of this problem, German patent, DE 10 2010 003 709 A1 describes a measuring transducer, in whose housing a drying chamber is arranged for collecting moisture, which has penetrated into the housing. For this, the drying chamber contains a moisture adsorbing material, or is composed essentially of a moisture adsorbing material. In the case of the described measurement transducer, at least one section of the inner space of the reference pressure supply extending in the interior of the housing is in humidity permeable connection with the moisture adsorbing material of the drying chamber. The reference pressure supply is embodied for this e.g. as a line, which has a line interruption, an opening or a humidity permeable wall, via which moisture can escape from its inner space. This moisture is then absorbed by the moisture adsorbing material of the drying chamber, which can be arranged directly bordering the line interruption, the opening or the humidity permeable wall or separated therefrom by a hollow space and/or a humidity permeable wall.
The drying chamber can only collect a limited amount of moisture, and is, consequently, preferably embodied as a replaceable module.
Since the drying chamber serves to collect moisture, which has already penetrated into the interior of the housing of the measuring transducer, it is arranged in the interior of the measuring transducer. It is, consequently, absolutely necessary to open the housing of the measuring transducer in the case of each replacement. That necessarily means an interruption of measurement operation. Moreover, in the case of opened housing, the entire interior of the housing is exposed to the surrounding air, so that in the case of a replacement in an environment with high humidity very rapidly very much moisture can penetrate into the housing, which must subsequently be collected by the newly installed drying chamber.
Moreover, the drying chamber requires space within the housing. Larger housings mean, however, greater quantities of air, and therewith, also more moisture. Moreover, the space requirement of the drying chamber is, in given cases, counter to a desired miniaturizing of the transducer housing, and a retrofitting of already existing measuring transducers with a drying chamber is usually not possible for reasons of space.